New Mexico Geological Society Annual Spring Meeting — Abstracts

Distinctive calcium-carbonate cementation features developed adjacent to and down-slope from several fault scarps cutting the early Pleistocene Sunport geomorphic surface south of Albuquerque, New Mexico. These features include calcified root mats with horizontal and vertical tubules and nodules, horizontal and vertical carbonate-cemented massive nodules and tubules, and massive carbonate-cemented sandstone beds with sharp bases. These cemented zones are localized in the hanging-wall blocks adjacent to the faults. The footwall sediments, fault, and sediments below the sharp base of these cemented zones are not cemented. Cementation extends a few meters to tens of meters horizontally away from the fault scarps and appears to be cuspate along the scarps. The carbonate cementation is distinct from stage II-III pedogenic calcium-carbonate horizons that later developed across the scarps.

We hypothesized that the cementation was caused by fault-related high-water-table seeps as water drained from the uplifted foot wall block to the hanging-wall block. We modeled the processes by assuming flow perpendicular to the fault from the footwall to the hanging wall. Flow rates were estimated using known permeability and porosity of the fluvial sands, permeability of fault gouge of variable thicknesses, and variable hydrostatic heads from the footwall to the hanging wall. The model water is saturated with calcium carbonate, which is then progressively precipitated in the shallow subsurface of the hanging wall by evapotranspiration (ET) and of CO₂. The distance from the fault to a steady-state no-flow boundary depends on the head, ET rate, and the thickness of the fault gouge. We assumed an initial "ballpark" ET rate of 40 inches per year (101.6 cm/yr). Using that ET rate and a fault gouge 1 cm thick, the model predicts that for a head of 3.5 m, the maximum distance that cementation is likely to extend from the fault is 34.8 m. This distance is farther for thinner fault gouge and shorter for thicker fault gouge. If the hydraulic conductivity of the fault gouge is greater than we assumed, then the cementation could have a greater extent than predicted by the model. If the fault gouge is less permeable, then the distance of cementation should be less.

We also estimated how long the cementation process could continue before all of the available porosity was plugged. Using an ET rate of 101 cm/yr, a fault gouge of 1.0 cm, the amount of time estimated to fill the pores of a pebbly sand with 30 percent porosity is about 206,000 years. Because the flow rate is faster with greater head, the time works out to be approximately the same for proportionate distances of the cementation zone for a given hydraulic conductivity.